Far from being a hodgepodge terrorism network, it now functions much like a state collecting taxes from people in areas under its control.

It also controls key infrastructure like power stations and oilfields in Syria and Iraq where it generates electricity to sell back to the Syrian government, The New York Times reported earlier this year.

ISIS is believed to control eastern Syria’s oilfields.Source:Supplied

It’s wealthy enough to pay its fighters a salary and has also collected millions in ransom from hostages, according to the US state department.

The group is also believed to have stolen millions of dollars from banks in Mosul after taking control of the area in June.

Australian National University’s Middle East terrorism expert Dr Rodger Shanahan said it’s impossible to know exactly how much cash ISIS has.

“Unless you’re privy to their books of account no one can definitely know. We know money has been going from the Gulf to Syria. We know they have been selling oil from some of the areas they control. We know they tax people, but how much comes from each, nobody will know except for them.”

However he added the strategy of expanding their reach is part of a plan to rival national governments in the region.

“If you call yourselves a caliphate you need to be able to do everything a government does, that includes infrastructure — the notion of holding dams is as much about claiming to be a government as it is about revenue raising.”

ISIS has claimed responsibility for beheading journalist James Foley in Syria.Source:AP

So could Australians filling up their cars with petrol be unwittingly funding the brutal regime?

Dr Shanahan said “there’s no real chance of that happening” as most refined fuel comes to Australia via Singapore.

“The relatively small amount they’re exporting is going to Turkey and Iraq ... local areas. They’re not getting mixed up with international exports … they’re not part of the international oil conglomerate.”

And regardless of how rich ISIS get, it’s likely to strengthen their brutal regime even further.

“It costs a lot of money to run thousands of fighters in the fields, supply them with ammunition,” Dr Shanahan said.

“If you can capture it you don’t have to buy it.”

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